1. Field of the Invention
This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature measurement circuits.
2. Description of the Related Art
Many digital systems, especially those that include high-performance, high-speed circuits, are prone to operational variances due to temperature effects. Devices that monitor temperature and voltage are often included as part of such systems in order to maintain the integrity of the system components. Personal computers (PC), signal processors and high-speed graphics adapters, among others, typically benefit from such temperature monitoring circuits. For example, a central processor unit (CPU) that typically “runs hot” as its operating temperature reaches high levels may require a temperature sensor in the PC to insure that it doesn't malfunction or break due to thermal problems.
Often, integrated circuit (IC) solutions designed to measure temperature in a system will monitor the voltage across one or more PN-junctions, for example a diode or multiple diodes at different current densities to extract a temperature value. This method generally involves amplifying a small voltage generated on the diode(s), and then subtracting voltage from the amplified temperature-dependent voltage in order to center the amplified value for conversion by an analog-to-digital converter (ADC). In other words, temperature-to-digital conversion for IC-based temperature measuring solutions is often accomplished by measuring a difference in voltage across the terminals of typically identical diodes when different current densities are forced through the PN junctions of the diodes. The resulting change (ΔVBE) in the base-emitter voltage (VBE) between the diodes is generally proportional to temperature. (It should be noted that while VBE generally refers to a voltage across the base-emitter junction of a diode-connected transistor and not a voltage across a simple PN-junction diode, for the sake of simplicity, VBE is used herein to refer to the voltage developed across a PN-junction in general.) More specifically, VBE may be defined as a function of absolute temperature by the equation
                              V          BE                =                  η          ⁢                      kT            q                    ⁢                                          ⁢          ln          ⁢                                          ⁢                                    I              C                                      I              S                                                          (        1        )            where η is the ideality factor of the PN junction, k is Boltzman's constant, q is the charge of a single electron, T represents absolute temperature, Is represents saturation current and IC represents the collector current. A more efficient and precise method of obtaining ΔVBE is to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔVBE may be related to temperature by the equation
                              Δ          ⁢                                          ⁢                      V            BE                          =                  η          ⁢                                          ⁢                      kT            q                    ⁢                                          ⁢                      ln            ⁡                          (              N              )                                                          (        2        )            where N is a constant representing a pre-selected ratio of the two separate currents that are supplied to the PN junction of the diode. FIG. 1 shows one example of a diode-connected transistor 110—a bipolar junction transistor (BJT) in this case—to which a current may be applied via current source 102 when switch 106 is open or via both currents sources 102 and 104 when switch 106 is closed. Current sources 102 and 104 may be configured such that a sum of the currents provided by both current sources is an integer multiple of the single current provided by current source 102. Thus, two separate currents in a predetermined ratio may be supplied to diode-connected BJT 110 in order to obtain a ΔVBE measurement.
In many systems, the diode or PN-junction may be configured at a remote location with respect to the measuring device, with the remote diode or PN-junction coupled to the measuring device via a tightly coupled twisted pair of wires or shielded pair of traces on a circuit board. A typical system is illustrated in FIG. 2, where transistor 110 is remotely located with respect to current sources 102 and 104, and is coupled to the current sources via twisted pair wires 140.
One prevalent problem with this method of delivering current to the remote diode or PN-junction when performing temperature measurements is the effects of electromagnetic interference (EMI), more specifically, currents that may be induced by EMI in twisted pair wires 140. For example, in the configuration shown in FIG. 2, the path for a current 132—induced by EMI signal 122 in cathode wire 144—will be through transistor 108 to ground, as the alternate direction would be into current source(s) 102 (and 104), which acting as a high impedance node(s) will impede current flow in that direction. Since induced current 132 will only flow through transistor 108 and not through remote transistor 110, it will typically not result in a temperature measurement error. However, if an EMI signal 120 induces a current 130 in anode wire 142, induced current 130 will travel through remote transistor 110, cathode wire 144 to transistor 108, and finally through transistor 108 to ground. Induced current 130 will take this path due to the high impedance presented at the other end by current source(s) 102 (and 104), which similarly determined the path for induced current 132. Induced current 130, which is generally an AC current, flowing through remote transistor 110 will typically get rectified and produce an error in temperature measurement by effectively modifying the preset current ratio between the two different currents provided to remote transistor 110 by current source 102 and combined current sources 102 and 104, respectively.
A common solution to the EMI problem described above has been to couple a large capacitor (typically in the range of 2200 pF) across remote diode-connected transistor 110. The large capacitor typically shunts EMI signal 120 (and/or EMI signal 122) away from remote transistor 110, keeping the current through transistor 110 substantially constant, thereby preventing EMI induced measurement errors. However, every time the current provided to remote transistor 110 is switched, the capacitor slows down the settling time of the VBE signal developed across the terminals of diode-connected transistor 110, necessitating a longer sampling period for sampling the VBE signal. Slower sampling frequencies in turn lead to longer conversion times and increased power consumption of the measuring devices. In addition, a shunting capacitor typically provides a finite amount of filtering, which may not be sufficient to enable completely accurate measurements. Therefore, it is desirable to develop a system and method to substantially eliminate EMI induced errors while maintaining fast sampling frequencies, thereby providing better temperature measurements.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.